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NASA's AIRS imagery revealed that Nepartak had some high, strong thunderstorms in the center of its circulation on October 12 at 11:35 a.m. EDT. The storm started to develop an elongated shape indicating transition into more of a frontal system.
Credit: NASA JPL, Ed Olsen
Tropical Storm Nepartak Becoming Extra-Tropical at Sea

Tropical Storm Nepartak is now speeding in a northeasterly direction in the northwestern Pacific Ocean, where it is becoming extra-tropical and developing frontal qualities.

The last official position of Napartak from the U.S. Navy's Joint Typhoon Warning Center was on October 13 at 6 a.m. EDT, when the storm was 605 nautical miles east-northeast of the island of Chichi Jima, Japan. That's near 31.5 North and 154.3 East. Nepartak was speeding to the northeast at 33 mph (29 knots) and it had maximum sustained winds near 46 mph (40 knots).

Animated multispectral imagery showed Nepartak was beginning to develop frontal characteristics, which is a sign that the circulation will soon fade. Before it becomes a front, however, it is expected to continue transitioning into an extra-tropical storm, and dry air in the mid-levels of the atmosphere is helping that happen.

The Atmospheric Infrared Sounder (AIRS) flies on NASA's Aqua satellite and captures infrared images of tropical cyclones take the temperatures of thunderstorm's cloud top temperatures to determine their strength. Aqua and AIRS flew over Nepartak on October 12 at 11:35 a.m. EDT, and noticed that the storm was already starting to take on an elongated shape, indicating a transition to more of a frontal system.

How does infrared imagery know how high clouds are in the sky? The coldest ones are higher in the sky (because in the troposphere, the lowest layer of atmosphere where weather happens, temperatures fall the higher up you go until you get to the stratosphere).

The highest clouds are as cold as or colder than 220 Kelvin or minus 63 degrees Fahrenheit (F) and second highest level of clouds are about 240 Kelvin, or minus 27F. The colder the clouds are, the higher they are, and the more powerful the thunderstorms are that make up the cyclone. Although Nepartak still had some strong convection, it is expected to wane soon with the transition into extra-tropical status. Nepartak is forecast to continue speeding northeast in the next day or two, when the transition should be complete and Nepartak will be no more.

Text credit: Rob Gutro, NASA/Goddard Space Flight Center

October 9, 2009

> View larger imageMicrowave images are created when data from NASA's Aqua satellite AIRS and AMSU instruments are combined. The cold areas in this image (yellow-green) indicates where there is precipitation or ice in the cloud tops. The microwave image suggests cold, high thunderstorms. The coldest area is in the storm's center (blue rounded area). Credit: NASA JPL, Ed OlsenNASA Takes a Microwave Look at Tropical Depression Nepartak

NASA's Aqua satellite flew over the latest tropical depression in the Western Pacific Ocean and looked at the storm called "Nepartak" using instruments that use microwave technology. The Atmospheric Infrared Sounder (AIRS) instrument on Aqua captured infrared, visible and microwave imagery of Nepartak and showed where the strongest thunderstorms were located.

Tropical Depression Nepartak had maximum sustained winds near 30 knots on October 9 at 11 a.m. EDT. It was located in the Western Pacific Ocean, about 290 miles south-southeast of the island of Iwo To (formerly Iwo Jima), near 20.3 North and 142.5 East. It was moving north-northwest near 5 mph, and was generating waves up to 12 feet high.

The U.S. Navy's Joint Typhoon Warning Center forecasts tropical cyclones in that region. Forecasters there reported that "Recent animated infrared imagery (like that from the AIRS instrument) shows spotty areas of deep convection wrapping towards a low level circulation center. The largest area of convection (and thunderstorms) is located on the eastern quadrant of Nepartak.

Infrared imagery measures temperatures and not only can it see cold, high cloud tops in tropical cyclones, but also the warm ocean waters that power the cyclones (if the sea surface temperatures are over 80F). Cold cloud top temperatures provide clues about the power of the thunderstorms in a tropical cyclone. The colder the clouds are, the higher they are, and the more powerful the thunderstorms are that make up the cyclone.

AIRS data is also coupled with data from the Advanced Microwave Sounding Unit (AMSU) that flies with AIRS on Aqua to create microwave images of storms. The AMSU image uses the radiances of the 89 GHz channel, and the cold areas in those images indicate where there is precipitation or ice in the cloud tops.

Nepartak is forecast to swing north and turn north-northeast over the weekend, passing east of Iwo To and Chichi Jima. Those islands are likely just to experience high surf as the center of Nepartak stays far off-shore.